The present invention relates generally to fabrication of objects and prototypes through the sequential deposition of material. More particularly, the invention relates to ultrasonic consolidation.
Numerous manufacturing technologies for producing objects by sequentially adding material exist, with the casting of liquid metal being perhaps the oldest such technique. In the past two decades, various processes for fabricating objects to net shape solely through material addition, i.e. without a finishing step such as machining to produce detailed, high-precision features, have been patented and, in a few cases, commercialized.
Most of these additive manufacturing processes either rely on an adhesive, or a solidification process in order to produce a bond between previously deposited material and each incremental volume of material which is added. Although the use of adhesives is convenient, the properties of the adhesive control the properties of the finished object, and this limits the usefulness of such processes in the production of engineering parts and products.
Processes which use solidification transformations result in objects with relatively uniform physical and mechanical properties, because the liquid which is present as each volume of material is added wets the previously deposited material, effectively acting as an adhesive with properties identical to those of the bulk material.
The most commercially successful of these technologies is stereolithography, in which a focused light source (typically an ultraviolet laser) is used to solidify a liquid photocuring polymer. As the laser focal point travels through a vat of liquid polymer, the polymer locally solidifies, and eventually, through appropriate programming of the motion of the focal point, a solid object is built.
Selective laser sintering is another additive manufacturing process in which a laser beam is used. In this process, a bed of solid powder is locally melted by a laser beam traversing over it. The partially melted powder aggregates, producing an object. Direct metal deposition is an improvement over selective laser sintering, and it is the subject of intense research and development around the world. In essence, the process involves the injection of metal powders into a high-power laser beam, while the laser is rastered across a part surface. The powders are melted in the beam, and deposited primarily under the influence of gravity.
Particularly with regard to the production of metal objects, prior-art methods require the presence of liquid metal. Various approaches to the problem include three-dimensional shape melting or shape welding, as described by Edmonds, U.S. Pat. No. 4,775,092, Doyle et al., U.S. Pat. No. 4,812,186, and Prinz et al., U.S. Pat. No. 5,207,371, and laser melting and deposition of powders as described in Lewis et. al., U.S. Pat. No. 5,837,960. Brazing of laminated objects, and closely related to it, infiltration of a low-surface tension and low-melting point alloy to fill voids in objects made by compacting or printing metal powders have also been employed. All of these processes require high temperatures and formation of liquid metals to produce a metal part.
More recently, nickel vapor deposition has been employed as a means of producing nickel shells for net-shape fabrication applications, U.S. Pat. No. 5,470,651. Nickel vapor deposition (NVD) allows thicker shells to be produced as vapor deposition rates are higher than in electroforming (Milinkovic, 1995). However, NVD involves the use of highly toxic gases and requires a specialized reaction chamber. The cost and risk of this technology are both very high.
The presence of liquid metal in a process presents numerous safety and material handling problems. Furthermore, the higher the melting point of the material, the greater these difficulties become. When low melting-point materials, such as solders or tin-based alloys, are used these issues are relatively insignificant. But when engineering materials such as iron, nickel or aluminum-based alloys are employed, these difficulties become important. Safety hazards include fumes, the possibility of metal breakout when reservoirs of liquid metal are required, and high-intensity energy sources and high voltage, when methods such as laser metal deposition, or shape melting are employed. When metal powder is used as a feedstock, as in laser metal deposition, the danger of explosion is very real. Other processes, such as metal spraying to produce net shape objects, result in powder generation as a waste product which may also present an explosion hazard.
The presence of liquid metal in additive manufacturing processes may also detrimentally effect dimensional accuracy of a part when built. The dimensional changes which occur during the liquid-solid transformation in metals are not wholly consistent, and are subject to random noise. This noise results in unpredictable and uncontrollable dimensional inaccuracies in parts built using liquid metal processes, the errors being of the order of 0.001 in to 0.005 in per inch. As part size increases, the errors accumulate, making it impossible to produce accurate parts. Several solutions have been proposed, including the use of a second, subtractive step for addressing the accuracy issues. However, this adds time, cost, and complexity to the process.
Selective laser sintering and laser-aided direct metal deposition are examples of processes which rely on thermal energy which is remotely generated, and transmitted through the object undergoing consolidation to produce a bond. Thermal energy is generated by a laser beam, and transmitted to the metal powder, either in the beam, as in the laser engineered net shaping process, or at the powder bed, as in selective laser sintering, and eventually reaches the location where bonding/consolidation of the growing object occurs.
The transmission of thermal energy results in a number of undesirable side effects. First the process is inefficient in that much more energy must be produced than is needed to produce a joint. Second, the thermal energy is not transported only to the location where it is used. Large volumes of additional material are also heated leading to problems such as residual stresses, curling, the need to cool objects, etc., which have been identified by other inventors. Dimensional accuracy is also difficult to control.
The forging processes now in use for rapid prototyping and tooling generally involve the use of metal powders which are densified under heat, pressure, or both. For example, hot isostatic pressing (HIP) is widely used in the aircraft engine industry. In this process, metal powders are compacted in a can which is subjected to high temperature and pressure. The material creeps to densify fully in the solid state. Cold isostatic pressing, i.e., isostatic high pressure compaction of powders at ambient temperatures can be employed for materials such copper, aluminum and low melting-point alloys. By producing a can of an appropriate shape, a near net-shape object can be formed. Powder forging can also be employed. Powder forging is a high temperature and pressure process conducted in a press in which the load is applied axially to the part rather than isostatically. A drawback of these processes is that they require some form of pattern or tooling, such as a can with a desired shape, or in the case of powder metal tooling, a ceramic or metal mandrel, against which the powders are forged.
The only commercialized low-temperature process for additive manufacturing of engineering scale metal components is electroforming, or plating. This is a very mature technology, which has recently been used to produce shells on near net-shape patterns for objects, usually tooling inserts for injection molding. Electroforming is a very slow process, as it takes up to two weeks to produce a shell 0.25 inches thick in a material such as nickel, which has sufficient strength and wear resistance to be used in permanent tooling. As a result, in rapid prototyping applications, this process is used only to create shells which require backfilling by some secondary material. Metal-powder filled epoxies are most often used, however, ceramic slurries, other plastics, cements, and low-melting point metals have also been used.
As a near net-shape forming technology, electroforming has other drawbacks besides extremely low deposition rate. In the electroforming process, metal salts are dissolved in an aqueous solution. When an electrical current passes through this bath, metal is deposited on the negatively charged surface, which, in net shape electroforming applications such as tooling, is a model which is the inverse of the desired final shape. Aqueous solutions of metal salts are generally toxic. Sludge forms in these baths as a by-product of the process. Both the liquid and the sludge are hazardous materials which must be handled and disposed of properly.
More recently, nickel vapor deposition has been employed as a means of producing nickel shells for net-shape fabrication applications, U.S. Pat. No. 5,470,651. Nickel vapor deposition (NVD) allows thicker shells to be produced as vapor deposition rates are higher than in electroforming (Milinkovic, 1995). However, NVD involves the use of highly toxic gases and requires a specialized reaction chamber. The cost and risk of this technology are both very high.
Electroplating or direct metal deposition are also used in the prior art for making objects with functionally gradient materials. Functionally gradient materials are those in which material composition is varied, whether rapidly or gradually, in order to allow a single component to more efficiently meet engineering service requirements. Examples in which such materials are used include injection-molding tools in which a copper zone is co-fabricated with the tool steel, in order to improve heat transfer in certain locations. Other examples are found in the hot zones of turbine engines and rocket motors, where it may be desirable to have a gradual gradient between a metal and a ceramic, or a metal and an intermetallic compound, so that certain areas of a part feature enhanced heat resistance, while others have excellent ductility.
Fabrication of functionally gradient materials often presents difficulties, because the materials may be metallurgically incompatible in the case of rapid variation, or because certain compositions may be very difficult to fabricate in the case of gradual variation. When copper and steel, for instance, are joined by prior-art fusion approaches, they tend to crack.
This invention is directed to a system and a method of fabricating an object by adding material layers incrementally and consolidating the layers through the use of ultrasonic vibrations and pressure. The layers are placed in position to shape the object by a material feeding unit. The raw material may be provided in various forms, including flat sheets, segments of tape, strands of filament or single dots cut from a wire roll. The material may be metallic or plastic, and its composition may vary discontinuously or gradually from one layer to the next, creating a region of functionally gradient material. Plastic or metal matrix composite material feedstocks incorporating reinforcement materials of various compositions and geometries may also be used.
If excess material is applied due to the feedstock geometry employed, such material may be removed after each layer is bonded, or at the end of the process; that is after sufficient material has been consolidated to realize the final object. A variety of tools may be used for material removal, depending on composition and the target application, including knives, drilling or milling machines, laser cutting beams, or ultrasonic cutting tools.
The consolidation is effected by ultrasonic welding equipment, which includes an ultrasonic generator, a transducer, a booster and a head unit, also called a horn or sonotrode. Ultrasonic vibrations are transmitted through the sonotrode to the common contact surface between two or more adjacent layers, which may include layers next to each other on the same plane, and/or layers stacked on top of each other. The orientation of the sonotrode is preferably adjusted so that the direction of the ultrasonic vibrations is normal to the contact surface when consolidating layers of plastic material, and parallel to the contact surface when consolidating layers of metal.
The layers are fed sequentially and additively according to a layer-by-layer computermodel description of the object, which is generated by a computer-aided design (CAD) system. The CAD system, which holds the layered description of the object, interfaces with a numerical controller, which in turn controls one or more actuators. The actuators impart motion in multiple directions, preferably three orthogonal directions, so that each layer of material is accurately placed in position and clamped under pressure. The actuators also guide the motion of the sonotrode, so that ultrasonic vibrations are transmitted in the direction required through the common contact surfaces of the layers undergoing consolidation.
In different embodiments, the system and method may incorporate the use of support materials to provide suitable substrates for any features of the object, which, when viewed sectionally, are overhanging. A description of the support resides in the CAD system, enabling the support to be built sequentially and additively. The support is preferably composed of less valuable material which is removed by stripping, cutting, dissolution, or by melting, when material having a lower melting-point than that of the object is used. As examples, useful support materials include ceramics, particularly water-soluble ceramics, and metal foils which do not bond but can compress and hold the up the build portion. The support materials may be consolidated using the same power supply and different joining parameters, though not every layer of the support need be bonded to the next layer, nor does the support need be fully consolidated. Indeed, weakly or partially bonded support material may be removed by breaking it up and shaking it loose using ultrasonic vibrations of appropriate frequency.
Other embodiments of the invention are directed to fabricating fiber-reinforced composites, including composites with continuous ceramic fibers in a metal matrix. According to one aspect, a layer of fibers is covered with a layer of a metallic powder, the surface of which is then partially consolidated by sweeping the surface with laser beam. Full consolidation is effected by the sonotrode of the ultrasonic welder. In a different embodiment, a layer of metallic foil is fed on top the powder and the sonotrode is used to fully consolidate the underlying structure.
Another aspect is directed to fabricating an object by tape lay-up. Tape from a spool is fed and cut into segments to create successive sections of the object, the direction of the tape segments preferably alternating between two orthogonal directions from section to section. The sonotrode is preferably positioned to consolidate the horizontal surfaces between the sections and the vertical surfaces between adjacent segments of tape on the same section.
Material may also be provided in the form of wire or strip fed from a spool. Such a configuration is particularly applicable to repairing and overhauling worn or damaged region of an object, wherein a sonotrode having a cutting tip is used to separate a dot of material and ultrasonically welds it in place.